Commercial synthetic silicas which have been ground by steam jet or air jet such as, for example, precipitated silicas, have bulk densities of from 50 to 90 g/l and a drying loss of 2 to 8% by weight, depending on the conditions of production or storage. For many applications it is necessary to lower the water content to less than 1% by weight through well-known drying processes. However, some of these drying processes have a loosening effect on the silica powder, i.e., during drying the bulk density is lowered to a value of between 30 and 40 g/l. Subsequent measuring out and packing of the precipitated silica is possible only with difficulty because of its consequently greatly increased volume. The dried silica should therefore be compressed to a higher bulk density.
It is well known that powdered substances such as, for example, synthetic silicas can be compressed by means of drum compressors, compressor screws, press band filters and/or other devices. However, these devices have the disadvantage that bulk densities in the range of from 50 to 100 g/l cannot be achieved or are not reproducible. The compressed powders usually show undesirable inhomogeneities such as nodules or similar undesirable components. In many cases the compressed powder cannot be loosened up again and is thus in the form of scabs, lumps or clods. What is more, the known devices are costly and susceptible to wear.